Electrical resistance heaters for indirectly heated thermionic cathodes in general produce stray magnetic fields which can adversely affect the operation of the cathode and in turn the operation of an electron tube into which the cathode is incorporated.
If the heater is to be operated from a source of AC voltage, the resulting AC magnetic field in the regions surrounding the heater will cause modulation of the electron flow in the region near the cathode. The result can be spurious signals, poor focusing of the electrons into a beam in beam-type tubes, and possible increases in beam interception on unwanted parts of the tube.
In the case of DC operation of the heater, the stray magnetic field distorts the path of the electrons in the regions where it is present, requiring that it be taken into account in designing electron beam optics. Moreover, since DC power supplies can be expected to show fluctuations in output voltage with changing load conditions on the power mains, etc., even a DC operated heater can be expected to produce some AC magnetic field components.
In order to combat these effects of stray magnetic field resulting from heater current, the prior art has resorted to the use of bifilar heater constructions in which the heater was wound of double conductor wire with one of the conductors being used as a current supply lead and the other a current return path. Thus, at each point on the heater winding the equal currents would be balanced, each one cancelling the stray magnetic field produced by the other.
Although this bifilar construction has proven excellent as far as the cancellation of stray magnetic fields, it has brought with it several disadvantages of its own. Specifically, the placement of the current supply and return conductors very close together for good magnetic field cancellation has sometimes resulted in accidental touching of the two conductors producing a destructive short circuit. Since it is common practice in many types of electron tubes to imbed the heater in a suitable sintered refractory potting compound, such short circuits have occurred all too often as the result of the pressures exerted on the heater during the process of forming a "compact" of refractory powder around the heater prior to sintering.
Additional failures of such bifilar heaters have occurred when they are operated from DC power supplies. These failures resulted from the development of electrically conductive paths through the refractory insulation material, produced by electrolysis of the refractory material or one of its impurities under the influence of high temperatures and the (uni-directional) electric field between the closely spaced bifilar conductors.
In many cases these problems associated with bifilar construction have resulted in its discontinuance, stray magnetic fields being then suppressed by either spacing the heater farther away from the cathode, or providing a layer of high temperature magnetic material such as cobalt between the heater and cathode to shield the latter from the stray magnetic field.
While the constructions utilizing a heater spaced far enough away from a cathode to avoid the influence of stray magnetic fields have been more or less satisfactory, they are not as efficient in terms of heater size and power consumption as when the heater can be located in close proximity to the cathode.
The provision of magnetic shielding material between heater and cathode is critically limited by the fact that no satisfactory material exists for operation in the range above 1000 degrees C., the Curie temperature of cobalt. Since dispenser type cathodes are usually operated above 1000 degrees C., cobalt is not a satisfactory magnetic shielding material.